Echoes affect communication sound quality. In general, echoes include short echoes and long echoes. When a remote voice is played by a local speaker, sound waves sent from the local speaker may be directly transmitted to a microphone (i.e., short echoes), or may be first reflected indoors and then transmitted to a microphone (i.e., long echoes). When an echo is sent back from the local microphone to a remote end, communication quality is noticeably degraded to result in a disturbing communication.